The German pilot who crashed a plane in the French Alps last week, killing 150 people, told officials at a Lufthansa training school in 2009 that he had gone through a period of severe depression, the airline said on Tuesday.

The statement is potentially damaging for the airline and its CEO Carsten Spohr, who told reporters last week that the carrier knew of no reason why 27-year-old Andreas Lubitz might deliberately crash a plane.

The fact that Lufthansa officials were aware that Lubitz suffered from depression raises questions about its screening process for pilots as it faces the threat of legal action from relatives of the victims.

Lufthansa said Lubitz broke off his pilot training for a period of several months but then passed medical checks confirming his fitness to fly.

When Lubitz resumed training in 2009, he provided the flight school with medical documents showing that he had gone through a “previous episode of severe depression,” Lufthansa said, citing emailed correspondence between Lubitz and the flight school.

Duesseldorf state prosecutors said on Monday Lubitz had been treated for suicidal tendencies before getting his pilot's licence.

In pictures: Germanwings Airbus A320 plane crash

In pictures: Germanwings Airbus A320 plane crash

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Flowers are left in front of the monument in homage to the victims of Germanwings Flight 4U 9525 in Le Vernet, southeastern France

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Andreas Lubitz was deemed ‘unsuitable for flight duties’ for a period of time during his training with Lufthansa and was receiving regular treatment for depression, sources have claimed as investigators focus their inquiry on his personal life and background

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People believed to be relatives of the deceased crew on Germanwings flight 4U9525 comfort each other at a reception centre in Le Vernet, France

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Rescue workers gather with friends and relatives of those killed onboard Germanwings flight 4U9525 at a reception centre in Le Vernet, France

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Flags representing some of the nationalities of the victims are seen as family members and relatives gather near the crash site of an Airbus A320 in the French Alps

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German and Spain flags symbolizing some of the nationalities of the victims are seen as family members and relatives gather for a ceremony in Le Vernet near the crash site of an Airbus A320 in the French Alps

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Flags symbolizing some of the nationalities of the victims, are seen near the memorial stele in Le Vernet during a ceremony to pay tribute to the victims of an Airbus A320 in the French Alps

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French gendarmes and investigators make their way through debris from wreckage on the mountainside at the crash site of an Airbus A320, near Seyne-les-Alpes

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French gendarmes and investigators work amongst the debris of the Airbus A320 at the site of the crash, near Seyne-les-Alpes, French Alps

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French gendarmes and investigators make their way through the debris of the Airbus A320 at the site of the crash near Seyne-les-Alpes, French Alps

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Wreckage of the Airbus A320 is seen at the site of the crash, near Seyne-les-Alpes, French Alps

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A body of a victim is evacuated by a French Gendarmerie rescue helicopter from the crash site of an Airbus A320, near Seyne-les-Alpes

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Rescue workers recover bodies of victims from the crash site of an Airbus A320, near Seyne-les-Alpes

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French military personnel work amongst the debris of the Airbus A320 at the site of the crash, near Seyne-les-Alpes, French Alps

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A helicopter of the French Gendarmerie flies over the crash site of the Germanwings Airbus A320 that crashed in the French Alps, above the town of Seyne-les-Alpes, southeastern France

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Search and rescue workers make their way through debris at the crash site of the Germanwings Airbus A320 that crashed in the French Alps, above the town of Seyne-les-Alpes, southeastern France

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Flowers and lit candles are placed on the ground in Cologne Bonn airport

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Andreas Lubitz, the co-pilot that crashed the Germanwings Airbus A320 in the French Alps

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Buses loaded with relatives of victims are escorted after their arrival at Marseille airport, southern France

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Family and relatives of the victims of the Germanwings plane crash in the Alps are taken on bus to the Prat airport from a hotel in Castelldefels in Barcelona, Spain, to take a Lufthansa flight to visit the crash site in Seyne les Alps in France

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An Airbus plane of German airline Lufthansa carrying onboard relatives of the Germanwings plane crash victims takes off from the Duesseldorf airport in Duesseldorf, western Germany, en route to Marseille

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A Germanwings employee places flowers in commemoration of the victims of the Germanwings plane crash in the French Alps, at the airport in Duesseldorf, Germany

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Members of German Government Chancellor Angela Merkel, Vice Chancellor and Economy Minister Sigmar Gabriel and Foreign Minister Frank-Walter Steinmeier, from right, hold a minute of silence to commemorate the victims of the Germanwings plane crash in the French Alps at the parliament Bundestag in Berlin, Germany

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Realtives of passengers of the Germanwings airliner that crashed in the French Alps leave the Gran Hotel Rey Don Jaime towards Barcelona El Prat airport where a lufthansa plane will fly to Marseille, in Barcelona, Spain

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Olivier Cousin (R), director of the mountain rescue team, which is responsible for the safety of the emergency workers on site, gives an interview in Seyne Les Alpes, France

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Alpine climbers take off in a police helicopter in Seyne Les Alpes

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German Chancellor Angela Merkel (L), French President Francois Hollande (C) and Spain's Prime Minister Mariano Rajoy pay respect to victims in front of the mountain in Seyne-les-Alpes, the day after the air crash of a Germanwings Airbus A320

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A member of the search and rescue personnel stands at the crash site of the Germanwings Airbus A320 in the French Alps

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Search and rescue personnel at the crash site of the Germanwings Airbus A320 in the French Alps

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The voice data recorder of the Germanwings jetliner that crashed in the French Alps

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The voice data recorder of the Germanwings jetliner that crashed in the French Alps

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Germanwings employees cry as they place flowers and lit candles outside the company headquarters in Cologne Bonn airport

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A student who knew some of the German students involved in a crashed plane, reacts during a minute of silence in front of the council building in Llinars del Valles, near Barcelona, Spain

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Photograph of victims, flowers and candles stand outside the Joseph-Koenig-Gymnasium high school where pupils had gathered to pay tribute to 16 students and two teachers from the school who were on Germanwings flight 4U9525 that crashed yesterday in southern France on March 25, 2015 in Haltern, Germany

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Pupils gather at the Joseph-Koenig-Gymnasium high school to pay tribute to 16 students and two teachers from the school who were on Germanwings flight 4U9525 that crashed yesterday in southern France in Haltern, Germany

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Students gather in front of the Josef-König secondary school in Haltern am See, western Germany, where some of the Germanwings plane crash victims studied

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A Lufthansa employee signs in a condolence book in Frankfurt, Germany

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The flags of (L-R) Aragon, Spain and the European Union are lowered to half-mast at the Regional Assembly of Aragon in Zaragoza, Spain, as a sign of respect for the victims of the German plane crash in the French Alps

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The German and the European Union flags hang at half mast in memory of the victims of the plane crash in France in front of the Federal Chancellery in Berlin, Germany

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French President François Hollande with Spain’s King Felipe VI and Queen Letizia pay their respects to the victims of the German plane crash in the French Alps

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Debris from the Germanwings Airbus A320 at the crash site in the French Alps above the southeastern town of Seyne

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Search and rescue personnel at the crash site of the Germanwings Airbus A320 in the French Alps

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Wreckage and debris lie on the mountain slopes after the crash of the Germanwings Airbus A320 over the French Alps

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Staff members of Germanwings and Lufthansa hold a candlelight vigil outside their headquarters in Cologne

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A photo provided by the French Gendarmerie shows the crash site in the French Alps

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A rescue helicopter from the French Gendarmerie flies over the French Alps, as day fades into night near to the crash site of the Airbus A320

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A general view of the crash site of a Germanwings Airbus A320 in the French Alps above Seyne-les-Alpes is pictured in this photo provided by the French Gendarmerie

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Family members of people involved in a crashed plane arrives at the Barcelona airport in Spain

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Rescue helicopters from the French Gendarmerie and the Air Force are seen in front of the French Alps during a rescue operation near to the crash site

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A helicopter of the French National Gendarmerie is seen in Seyne, south-eastern France, near the site where a Germanwings Airbus A320 crashed in the French Alps

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French emergency services workers (back) and members of the French gendarmerie gather in Seyne, south-eastern France, near the site where a Germanwings Airbus A320 crashed in the French Alps

BORIS HORVAT/AFP/Getty Images

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An helicopter of civil security services is seen in Seyne, south-eastern France, near the site where a Germanwings Airbus A320 crashed in the French Alps

ANNE-CHRISTINE POUJOULAT/AFP/Getty Images

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Relatives of passangers of the Germanwings plane crashed in French Alps arrive escorted by police officer at Terminal 2 of Barcelona El Prat airport in Barcelona

David Ramos/Getty Images

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Relatives of passangers of the Germanwings plane crashed in French Alps arrive at Terminal 2 of Barcelona El Prat airport in Barcelona, Spain

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Relatives of passengers killed in Germanwings plane crash arrive at the airport in Duesseldorf, Germany

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Police escort a family member of an aircrash victim at Barcelona's El Prat airport

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French firefighters prepare to take-off in Digne-les-Bains for the crash site of an Airbus A320, in the French Alps

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A family member of a passenger killed in Germanwings plane crash reacts as he arrives at Barcelona's El Prat airport

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People arrive at a holding area for friends and relatives of passengers on Germanwings flight 4U9525 from Barcelona to Dusseldorf at Dusseldorf International Airport in Dusseldorf, Germany

Sascha Steinbach/Getty Images

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People waiting for flight 4U 9525 are lead away by airport staff at the airport in Duesseldorf, Germany

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A relative (C) of passangers of the Germanwings plane crashed in French Alps arrives at the Terminal 2 of the Barcelona El Prat airport

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A man who appears to have waited for the missing flight 4U 9525 reacts at the airport in Duesseldorf, Germany

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epa04676936 A man looks at a monitor showing a map released on the webpage 'flightradar24 with the exact point where the radar signal of the crashed Airbus A320 aircraft operated by German budget airline 'Germanwings' went missing near Barcelonnette, in the department of Alpes-de-Haute-Provence, in Madrid, Spain

EPA

They last week found torn-up sick notes showing that Lubitz was suffering from an illness that should have grounded him.

Germanwings said it had not received a sick note from Lubitz for the day of the crash. Lubitz had a valid medical certificate at the time of the crash of the Airbus A320 operated by its budget unit, Lufthansa added.

Lufthansa said it had passed the email correspondence and additional documents to the Duesseldorf prosecutors after internal investigations.

Lufthansa was already facing unlimited liability for damages in the crash, lawyers have said, and has told its insurers to set aside $300 million to deal with claims, recovery costs and the loss of the aircraft.

For Germanwings to limit its liability, it would have to establish that it and its employees and agents were not in any way at fault or that the accident had been caused solely by the fault of a third party.

Pilots pass psychiatric screening ahead of starting training but annual medical checks do not include a specific in-depth psychiatric test. Pilots are expected to inform medical examiners at their annual checks of any health issues that could affect their flying ability.

Psychiatrists and pilots have said there is no fail-safe test to spot suicidal pilots, but lawyers representing some of the families of victims of the Germanwings crash called on Tuesday for more regular psychiatric testing of pilots.

French air accident investigators also said on Tuesday that their probe into the crash would look at psychological screening procedures for pilots and cockpit access, indicating the two areas where they are most likely to make safety recommendations that could affect the whole aviation industry.

“In these cases, when you're acting for families, they're keen to see that lessons are learned, that pressure is put on airline authorities to improve flight safety,” Jim Morris, a partner at law firm Irwin Mitchell, told Reuters.